1. Field of the Invention
The present invention is addressed to the removal of nonmetallic inclusions from molten metal and more particularly a multistage-electromagnetic purification method of molten metal, which relates to the field of casting technique.
2. Background
Nonmetallic inclusions present in molten metal, which are deleterious to the mechanical properties, structural integrity, machinability and surface quality of products made from the metal, should be removed prior to casting. Ceramic foam filters (CFFs) are commonly used in casthouses, but with low filtration efficiency and unstable performance. Deep bed filters or particle bonded filters have better performance than CFFs, but with high costs and maintenance difficulty. Moreover, pore size of all kinds of filters should be greatly decreased in order to capture micro-sized inclusions, however, accompanying with low penetration, high flow resistance and early blockage problem. By contrast, electromagnetic separation is a novel method for inclusion removal to meet the growing demand for cleaner metals. It makes use of the difference in electrical conductivity between inclusions and the melt, and inclusions are separated outwards and captured by the sidewall if electromagnetic force field is exerted directly or generated by induction in the melt enclosed in a separator pipe. Due to high efficiency of electromagnetic separation, the channel size of separator can be much larger than that of filter pores without sacrificing the capability of capturing micro-sized inclusions. Therefore, electromagnetic separation has higher inclusion removal efficiency than filtration, while electromagnetic separator has less flow resistance and longer life period of use than filters.
China Pat. No. 01,142,619.5, issued to Shu D. et al. on Oct. 6, 2004, discloses a method for removal of nonmetallic inclusions from aluminum melt by use of an induction coil and a ceramic separator with square channels, wherein the channel size of the separator can be enlarged to sustain high removal efficiency of micro-sized inclusions because the induced secondary flow of the melt helps transport the inclusions from inner region to the outside skin layer in the cross section. However, the described ceramic separator is a single-stage type with open channels, i.e., there are no changes in the shape and size of the cross section along the flowing direction. It is well known that electromagnetic force distributes unevenly inside the melt for inductive separators, generally decaying exponentially in the radial direction from the outer wall to the center in the cross section. Consequently, the central region of a single-stage separator is a “weak zone” for electromagnetic separation, because inclusions located in this zone migrates slowly towards the outer sidewall, especially in the case of a large-sized separator. Besides, the melt at the centerline of the separator is not subject to electromagnetic force by theory. As a result, inclusions located there cannot be separated, causing the existence of “dead zone”.
The existence of “weak zone” and “dead zone” of electromagnetic separation in a single-stage separator greatly hinders further increase of inclusion removal efficiency from bulk melt of high flow rates.